Circulator

A circulator is a passive non-reciprocal three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted to the next port in rotation (only). A port in this context is a point where an external waveguide or transmission line (such as a microstrip line or a coaxial cable), connects to the device. For a three-port circulator, a signal applied to port 1 only comes out of port 2; a signal applied to port 2 only comes out of port 3; a signal applied to port 3 only comes out of port 1, so to within a phase-factor, the scattering matrix for an ideal three-port circulator is

Depending on the materials involved, circulators fall into two main categories: ferrite circulators and nonferrite circulators.

Ferrite circulators are radio frequency circulators which are composed of magnetised ferrite materials. They fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on stripline are of the 3-port type. Two or more Y-junctions can be combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.

Though ferrite circulators can provide good 'forward' signal circulation while suppressing greatly the 'reverse' circulation, their major shortcomings, especially at low frequencies, are the bulky sizes and the narrow bandwidths.

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Wikipedia